Thicker main caps (like the HiPo), a "Hecho en Mexico" cast into the lifter valley and two knobs (or bumps) on the front of the block up towards where the heads mount. The knobs should be able to be seen without taking anything off.
PM Joe and he can educate you. I just provided the picture space many years ago when we were using them as tutorials on the old forum. Obvious stuff I notice as a builder is the thicker main web/cap and oil pan rail areas.
I guess it was around Christmas when he sent me the pix
If you're looking at the front of the block, you'll see two "knobs" just below the cylinder deck. One on each side. If the oil pan is off the block, you'll see thick main caps like on a HIPO 289 block. If the intake manifold is off, you'll see in the lifter galley a square cast in there that says "Hecho En Mexico." Also, they have unique casting numbers. The ones I've seen are "C8AM," "D1ZM," and "75YZ."